1. Field of the Invention
The present invention relates generally to piston assemblies, and more particularly, to a lightweight mud pump piston assembly.
2. Description of the Prior Art
Generally, mud pumps are used to pump slurry, mud solutions, oils, water and other fluids. In the oil field, mud pumps are often required to circulate drilling fluid through a borehole having a mud column thousands of feet in length. The internal pressures in the compression chambers of mud pump cylinders can often reach 2,000 pounds per square inch or more depending largely on the diameter of the bore of the mud pump cylinders and the depth of the well bore. Given a typical mud pump piston having a six inch diameter, a compression chamber pressure of 2,000 pounds per square inch results in a total force on the piston of approximately 56,000 pounds. This force is placed on the piston with each piston stroke.
In light of this heavy burden, the hub portion of the piston, which comprises the bulk of the piston assembly, has in the past been constructed out of high strength metals such as steel or similar metal alloys. Typical examples of solid steel hubs can be seen in U.S. Pat. No. 4,281,590.
Although such construction is quite suitable for handling the large force placed on the piston, this solution causes the piston assembly to have a substantial weight that exacerbates other problems which mud pumps have a tendency to experience.
Wear on the piston assembly and in the cylinder liner is one of the chief causes for downtime in a mud pump. A major cause of wear is the drilling fluid which is especially erosive when pumped at the high pressure and substantial volume that is required for drilling. The weight of the piston combined with the general wear of the cylinder liner resulting from the drilling fluid tends to create an unbalanced contact of the piston with the liner. Although the sealing elements which seal between the hub and the cylinder create a centralizing bias on the piston, this centralizing bias tends to be offset due to the effects of the weight of the piston. Furthermore, when the mud pump is positioned so that the cylinders are generally horizontal, the weight of the piston often results in greater contact of the piston on the bottom portion of the liner than the upper portion. These problems are further aggravated when the connecting rod to which the piston is secured is somewhat out of alignment due to bearing support wear or other pump problems.
These conditions generally cause an uneven wear pattern of the piston as well as the liner and, in some cases, will cause a metal to metal scouring of both the liner and any metal portion of the piston that may contact with the liner.
A number of pistons using light weight materials have been developed for various purposes. United Kingdom Patents 2,045,389 and 2,033,537 A show plastic pistons intended for use in air compressors. However, these snap on plastic pistons are designed for operating at much lower pressures than mud pump pistons and therefore do not suggest how it is possible to use such lightweight materials where the pressure demands on a piston are much greater.
U.S. Pat. Nos. 4,072,088 and 4,404,935 show how various lightweight and ceramic materials can be used to overcome problems specifically related to internal combustion engines such as heat and high revolutions per minute. These internal combustion pistons are generally shaped much differently than mud pump pistons and the problems these Patents address by using various lightweight materials are not subject to the same restraints in using light weight material in mud pumps. Therefore, these Patents do not suggest how it would be feasible, or even useful, to employ light weight materials in a mud pump piston.
Consequently, a need exists for improvements in piston assemblies which will result in greater reliability and dependability of operation of such assemblies.